In the semiconductor process industry, a large variety of different ultrapure gases are used in the fabrication plants. These gases include bulk gases such as N.sub.2, O.sub.2, H.sub.2 and Ar which are normally used in very large quantities, and specialty gases such as He, A.sub.s H.sub.3, PH.sub.3, SiH.sub.4, NH.sub.3 and NF.sub.3 which are used only in small quantities. In general, the bulk gases are used for purging of chambers, oxidation and cleaning of wafers, while the specialty gases are used as reactant or etching gases.
The bulk gases are normally stored in large storage facilities, for instance, N.sub.2 can be supplied from a liquified-nitrogen storage tank located in the gas yard or delivered from a pipeline from a remote air-separation plant; O.sub.2 and Ar can be supplied from liquified gas storage tanks; while H.sub.2 can be delivered from either a liquified-gas storage tank or a bank of high-pressure gas cylinders. The bulk gases are normally passed through purifiers and gas filters for removing impurities and contaminating particles before allowed to enter a gas-distribution piping system installed inside a cleanroom. On the other hand, the speciality gases are normally stored in small quantities in gas cylinders and are sent directly to the process tools from cylinders stored inside gas cabinets in the cleanroom. The gas cabinets are exhausted safety enclosures that contain the gas cylinders and the necessary gas handling equipment. The gas cabinets serves a major function of allowing purging and safe exchange of the speciality gas cylinders. The gas handling equipment, which includes gas panels incorporating all components for the control and monitoring of high purity gases. In most semiconductor fabrication facilities, the gas cabinet contains at least two process cylinders to allow easy switch-over when one cylinder is empty. In addition, another cylinder of inert gas such as nitrogen is provided for purging the piping line.
In most fabrication processes, the supply pressure for the bulk and the speciality gases is kept at under 10 Kg/cm.sup.2. A few exceptions exist such as chlorine and dichlorosilane. At each point of use, the pressure of the bulk or speciality gas has to be independently and locally controlled by a series of flow control valves, pressure regulators, pressure sensors and particle filters located inside a gas manifold box. The precise pressure required for each bulk or speciality gases to be delivered to a specific process tool is determined by the process requirement. In most semiconductor cleanroom facility, one or more gas manifold boxes are installed nearby to each process tool to facilitate gas distribution and control.
A typical bulk gas distribution system 10 is shown in FIG. 1. The gas distribution system 10 is used, for instance, to distribute an inert gas such as helium. The system 10 consists essentially of two gas supply sources, i.e., a main gas supply source 12 which is a trailer mounted gas source and a back-up gas supply source 14 which is a plurality of gas cylinders with their outlets 16 parallelly connected. The gas from the main gas supply source 12 is fed to a three-way flow control valve 18 through conduit 22. The conduit 22 further includes flow control valves, pressure regulators, pressure sensors and particle filters which are not shown for simplicity reasons. The back-up gas supply source 14 is also connected to the three-way flow control valve 18 through conduit 24 for feeding a gas to the process tool 20 through the three-way flow control valve 18 when the valve is manually switched over upon an indication that the pressure of the main gas supply source 12 has dropped to a level that requires replacement. The conduit 24 further includes flow control valves, pressure sensors, pressure regulators and particle filters which are not shown.
In the conventional gas supply system 10 shown in FIG. 1, the system functions properly as long as it is tended by a system operator to effectuate the manual switching of the three-way control valve 18 when necessary. The effective operation of the gas supply system 10 is entirely dependent on the attentiveness of the system operator and thus, even when elaborate warning devices are installed on the system control panel, it is possible that human error can lead to severe consequences when a gas is fed to the process tool 30 at insufficient pressure or no gas is fed to the process tool 30. As a result, a significant loss in the fabrication yield occurs.
It is therefore an object of the present invention to provide an apparatus for supplying an uninterrupted gas to a process tool that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for the automatic switching of a gas supply that can not be affected by human errors made by a system operator.
It is a further object of the present invention to provide an apparatus of an automatic switching gas supply system that includes a main gas supply source and at least one auxiliary gas supply source.
It is another further object of the present invention to provide an apparatus of an automatic switching gas supply system wherein the automatic switching function is accomplished by pneumatic power and functions without electric power supply.
It is still another object of the present invention to provide an apparatus of an automatic switching gas supply system consisting of a main gas supply source and a back-up gas supply source with a pressure differential of at least 0.5 Kg/cm.sup.2 between the two gas supply sources.
It is yet another object of the present invention to provide an apparatus of an automatic switching gas supply system in which a gas supply conduit switches automatically to a back-up gas supply source when the pressure of the main gas supply source drops below the pressure of the back-up gas supply source.
It is still another further object of the present invention to provide a method for automatic switching to a back-up gas supply in a gas supply system by providing a main gas supply source and at least one back-up gas supply source wherein a gas flow is controlled by a pressure differential between the two sources and by a three-way flow control valve.
It is yet another further object of the present invention to provide a method for automatic switching to a back-up gas supply in a gas supply system by flowing a gas through a conduit from the back-up gas supply source into a process tool when a pressure in the main gas supply source drops by at least 1 Kg/cm.sup.2 lower than the pressure in the back-up gas supply source.